The present invention relates broadly to the regeneration of activated carbon. More specifically, the invention pertains to solvent regeneration of activated carbon by the selective use of a regenerant solvent containing in solution the same chemical species which is sorbed on the carbon.
Activated carbon is widely used in industry as an agent for recovery, removal, and purification of broadly ranging chemical species from a fluid stream. In the normal use of activated carbon chemical species build up on the carbon causing a progressive reduction in the carbon's ability to remove additional chemicals from a fluid. Therefore, at certain intervals the carbon is regenerated, i.e. purged of a substantial portion of the adsorbed chemicals.
There are four general methods of carbon regeneration in current use: solvent wash, acid or caustic wash, steam reactivation, and thermal regeneration. Use of a solvent or acid or caustic wash is practiced by passing the solvent or wash through the carbon bed until the sorbed chemical species are removed. The carbon bed is drained of the solvent or wash to prepare the bed for reuse. Solvent regeneration can be unfeasible because very large amounts of solvent must be employed to recover the sorbed species from the bed and the large quantity of solvent must then be treated to recover the chemical species from the solvent. Acid or caustic wash forms a salt on the carbon bed which may be difficult to remove and to later separate from the solution.
Chemical species with low boiling temperatures can in certain cases be removed from carbon beds by steam. Typically 3-5 pounds of steam per pound of chemical species must be passed through the carbon opposite to the normal flow direction, and is vented to the atmosphere or condensed and recovered. The generation of live steam and condensation or other disposal of spent steam in the quantities required for steam regeneration is not economical. Also, the steam usually condenses to some extent into water, which dilutes the stream of chemical species recovered and increases the amount of distillate required to be treated.
The method most widely used today is thermal regeneration, which involves three basic steps: drying; baking or pyrolysis of chemical species; and activating, by oxidation of the carbon residues remaining after the baking step. This method requires drying temperatures of at least 212.degree.F., baking temperatures between 212.degree. and 1500.degree.F., and activating at a carbon temperature in excess of 1500.degree.F. The use of the high temperatures required in thermal regeneration is disadvantageous in that as much as 10 percent of the carbon may be lost in the regeneration cycle. Furthermore, large quantities of heat and special ovens must be provided to accommodate the heating. Also, time and efficiency are lost when the activated carbon is removed from the adsorber, conveyed to the oven, regenerated, reconveyed to the adsorber, and finally reinstalled.